In humans, 50% of all spontaneous miscarriages are due to non-disjunction errors at the first meiotic division, while 90% of Down syndrome cases can be attributed to errors in maternal meiosis I. The predominant cause of these errors lies with misregulation of the recombination events that define meiosis. Reciprocal recombination, or crossing over, occurs during prophase I and is essential for tethering homologous chromosomes together until the first meiotic division. Recombination is initiated by the formation of DNA double-strand breaks (DSB) that are then processed to form either crossovers (CO) or noncrossovers (NCO). CO frequency and placement is tightly regulated to ensure at least one CO per chromosome and to prevent closely spaced CO, a process known as interference, while selection of COs from a large pool of DSBs is subject to stringent crossover homeostasis. Both interference and crossover homeostasis are thought to arise early in the DSB repair process. Two CO pathways have been described in a number of organisms, including mammals. The class I pathway is regulated by the meiotic components of the DNA Mismatch repair (MMR) family (MSH4-MSH5 and MLH1-MLH3 heterodimers), while the class II pathway is regulated by MUS81-EME1. Studies in the PI's laboratory have determined that there is a degree of integration between the two pathways that appears to be unique to mammals, such that the Class II pathway leads to an increase in Class I crossover intermediates, as demonstrated by the increase in MLH1-MLH3 appearance on meiotic chromosomes. This increased flux through the Class I pathway maintains the final chiasmata count; presumably because these additional Class I events replace those Class II events that can no longer occur in the absence of MUS81. Alternatively, it is possible that a third crossover pathway is recruited to maintain the final chiasmata tally, since double mutants for both Mlh3 and Mus81 still retain residual chiasmata. In either case, this integration between CO pathways occurs late in prophase I, indicating a novel mechanism for monitoring final CO output that is temporally distinct from the earlier interference and homeostasis events. Studies in this proposal are aimed at understanding how this integration between the two pathways is achieved. Preliminary data presented herein point towards two possible mediators of these events: BLM helicase, and the newly-identified BTBD12 endonuclease, a putative target of the ATM kinase. Our overall hypothesis is that the choice of CO pathway may involve integrated signaling through two regulators, BLM and BTBD12, each of which serve either to provide the appropriate substrate for CO processing and/or to divert structures between pathways, as needed. The specific aims are: (1) to examine the role of BLM at different stages of meiotic prophase I in mammalian germ cells; (2) to understand the meiotic role of BTBD12 and how this function is regulated by ATM kinase; and (3) to explore the mechanisms by which the two CO pathways are integrated in late prophase I to produce the appropriate tally of chiasmata.